Various instruments and methods have been used to prepare adjacent vertebrae of the spine for insertion of a bone dowel or artificial implant. These instruments and methods allow for the removal of disc material from between adjacent vertebrae as well as a portion of each vertebra to form an implantation space or opening. Drill sleeve, tubular member, sheath, working channel, and guard are a few of the names for the protective guide used to guide drills and other disc and bone removal devices into the spine to form the implantation space for receipt of the bone dowel or artificial implant. Certain of the guides have a sharpened end or include teeth for engaging the vertebrae upon application of an impaction force. The guide can be positioned so as to span the disc space and be seated into the adjacent vertebrae to provide protected access to the spine during the process of forming the implantation space. The implantation space spans the disc space and protrudes into each of the adjacent vertebrae along two opposed resected arcs. An example of the procedure for drilling holes across a disc space and instrumentation pertaining thereto are described in U.S. Pat. No. 5,484,437 to Michelson and is incorporated herein by reference.
The use of cylindrical implants is desirable because the surgeon can prepare the recipient site by drilling a cylindrical hole across the disc space and into the adjacent vertebrae. The curved surface of the cylindrical holes drilled into the vertebrae provide for the possibility of tight congruency when the cylindrical hole is fitted with an implant having corresponding cylindrical portions of matched diameter.
Typically, threaded artificial implants, such as the implant disclosed in U.S. Pat. No. 5,015,247 to Michelson, the entire disclosure of which is incorporated herein by reference, are placed into the implantation space between the adjacent vertebrae to directly participate and be incorporated in the ensuing fusion. Threaded bone dowels, such as those taught by Viche may also be placed into the implantation space for the purpose of bridging the opening and to be incorporated into the fusion between the vertebrae. Moreover, artificial implants of the push-in type, such as those disclosed in U.S. Pat. No. 5,593,409 to Michelson and assigned to the assignee of the present application and incorporated herein by reference, may also be inserted into the implantation space formed by the above described instruments and methods.
Whether pushing or threading an implant or dowel into the implantation space, the surgeon attempts to orient the sheath for guiding the formation of the implantation space to remove approximately the same amount of material from each of the vertebra. Once the implant or dowel is implanted, a fluoroscope may be used to assist in determining if proper placement has occurred.
A number of different types of surgical navigation systems have been described that include indications of the relative positions of medical instruments and body parts used in medical or surgical procedures. For example, U.S. Pat. No. 5,383,454 to Bucholz; PCT Application No. PCT/US94/04530 (Publication No. WO 94/24933) to Bucholz; and PCT Application No. PCT/US95/12894 (Publication No. WO 96/11624) to Bucholz et al., the entire disclosures of which are incorporated herein by reference, disclose systems for use during a medical or surgical procedure using scans generated by a scanner prior to the procedure. Surgical navigation systems typically include tracking means such as for example an LED array on the body part, emitters on the medical instruments, a digitizer to track the positions of the body part and the instruments, and a display for the position of an instrument used in a medical procedure relative to a body part.
Procedures for preparing an implantation space in the spine present certain particular challenges to be addressed at specific levels within the spinal column. For example, performing such a procedure at L4-5 of the spine can raise the following issues: 1) During a posterior approach, significant muscle stripping and tearing is required to reach the L4-5 disc space. Significant post-operative trauma to the patient may result. 2) During an anterior approach, with either a transperitoneal or retroperitoneal, open or laparoscopic approach to the L4-5 disc space, the great vessels lie almost directly on the front of the spinal column at that level. Dissection and manipulation of these vessels may be time-consuming and difficult.
The applicant determined that a retroperitoneal “oblique approach” to the L-5 disc space can present a preferred surgical solution. The muscle splitting and tearing of the posterior approach is therefore not inevitable, since the oblique approach takes place just anterior to the psoas muscle located laterally along the vertebral column. The oblique approach also allows for an approach slightly posterior of the great vessels lying anterior on the spinal column, thus reducing any risk of injury to the great vessels.
In viewing the spine solely from a lateral fluoro image, the spine surgeon, via this oblique approach, would have a limited ability to see an indication of the depth and direction of their instrumentation as it moves in a posterior lateral fashion across the disc space towards the exiting nerve root.
In light of the foregoing, it would be beneficial in the art for a system and method for the placement of a guide in the spine that provides directional assistance to the surgeon to improve the placement of the guide used in forming the implantation space as well as for use in determining the depth of insertion of instruments passing through the guide, thereby improving the placement of implants and dowels into the implantation space.